<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script src="vue.js" type="text/javascript" charset="utf-8"></script>
</head>
<body>

  <div id="app">
    {{a}}
    <input type="text" v-model="a">
  </div>


  <script type="text/javascript">
    var data={a:1}
    var vm = new Vue({
      el: "#app",
      data: data
    })


    vm.$watch('a',function (newVal,oldVal){
       console.log(newVal,oldVal)
    })

    vm.$data.a = "test...."
  </script>
</body>
</html>
